Hi!
For now, I've bought this 4K Monitor : ASUS287Q.
There is a Displayport cable but no Mini Display port cable.
So, I had to buy a Mini Display port cable to plug the monitor to my Surface Pro 4 i7.
To get 60 Hz, use monitor Menu : Menu>System setup>DisplayPort Stream>DP12.
This monitor is not as bright as Surface Pro 4 screen.
I think I will switch to ASUS PA328Q in a few weeks.
Have a nice day!